There are different types of edge terminations used in metal-oxide semiconductor field-effect transistor (MOSFET) devices. For example, in conventional MOSFETs the edge termination consists of a set of floating field rings/field plates across which the potential drops in a step wise fashion from the source potential to the drain potential. Recently a new type of MOSFET, commonly known as Super Junction MOSFET (SJMOSFET), has been designed that employs order of magnitude higher drift layer concentration resulting in very low on resistance for a given breakdown voltage. This is accomplished by the incorporation of P type vertical junction regions in the core drift region. The field ring based edge termination used for a conventional MOSFET is deemed unsuitable for the SJMOSFET. Its breakdown voltage will be much lower than the core breakdown voltage. As such, different edge termination schemes are generally employed.
For example, one of the edge terminations used for the SJMOSFET is a source field plate running over a thick low temperature oxide (LTO) layer over the termination region. The source field plate together with the floating P columns underneath it supports the source drain potential. While this is an acceptable edge termination for the SJMOSFET and is used commonly, it has the drawback of causing electric arcing between the unexposed areas of the source metal field plate and the drain for breakdown voltage higher than the potential at which air breakdown takes place (around 400V). In order to avoid electric arcing between the source field plate and the drain, the field plate is covered with a passivation layer, such as silicon nitrogen (SiN). However, because of the brittle characteristics of SiN and also the sharp features of the etched metal field plate edges, passivation cracks occur leading to the generation of arcing to air. To avoid such arcing potential it is necessary to cover the metal with a crack free passivation layer.
Therefore, while there are advantages associated with Super Junction MOSFET devices, there are also disadvantages associated with them when it comes to the edge termination areas. As explained earlier, one of the disadvantages is that when a field plate is incorporated into a Super Junction MOSFET device, a thick oxide (e.g., approximately 5-6 micrometers thick for a 600V device) is utilized. It is also necessary to coat the field plate with passivation material, such as SiN and Polyimide in order to prevent electrical arcing between the edge of the metal field plate and the drain (the scribe line).